In imaging methods like electro(photo)graphy, magnetography, ionography, etc. a latent image is formed which is developed by attraction of so called toner particles. Afterwards the developed latent image is transferred to a substrate and fused on this substrate. In such methods dry radiation curable toner particles may be used. Dry toner particles are basically polymeric particles comprising a polymeric resin as a main component and various ingredients mixed with said toner resin. Apart from colourless toners, which are used e.g. for finishing function, the toner particles comprise at least one black and/or colouring substance, e.g., coloured pigment. Examples of such dry toners are described in European patents EP 1 756 675 B1, EP 1 930 780 B1, and EP 2 019 340 B1 in the name of the Applicant, which are included herein by reference.
To obtain a good curing efficiency the toner has to be brought in a low viscous state so that the mobility of the reactive groups (e.g. double bounds) is high and the right degree of crosslinking can be achieved using e.g. UV curing or electron beam curing. As the viscosity of the toner (at fusing temperature) is linked to the glass transition temperature (Tg), this means that the glass transition temperature should not be too high. This applies in particular for substrates with a dimensional stability which decreases at higher temperatures, such as thin polyethylene or thin polypropylene substrates. Using low viscosity toners has however some major drawbacks.
A first drawback is that the use of a low viscosity toner causes limitations with respect to storage conditions and an increased risk for the formation of toner aggregates or lumps in the developing unit during the toner carrier mixing. Under normal transport conditions, the temperature may be high, resulting in sintering of the low viscosity toners. To prevent sintering, the low viscosity toners may be transported under special conditions, but this increases the transport cost. Therefore, preferably prior art toners have a Tg which is typically larger than 50° C.
A second drawback is that during the mixing of toner and carrier in the developing unit the surface additives used to control the charge and toner flow characteristics will be easily embedded. This change in toner surface state changes the charging and flowing properties of the toner meaning that no stable charge over time and under different page coverage can be established. Another effect of embedded surface additives is that the develop ability decreases by a stronger interaction between the toner surface (typically a polymer surface) and carrier so that the adhesion forces increase and it is more difficult to develop the toner onto the photoconductor for the same development potential and to transfer the toner from the photoconductor to the substrate. This effect will be even more pronounced in high speed printing, i.e. at speeds higher than 0.3 m/s. Those problems can be overcome by applying high amounts of surface additives on the toner surface. This however will reduce the ability to fuse and as a consequence cure the toner in a proper way, because it is typically desirable that the toner particles first lose their corpuscular behaviour and form a continuous film prior to performing the crosslinking.
Another reason why most of the toners that are used in the field have a high Tg (and thus a high melt viscosity) is the storage stability of the printed material. Printed substrate material put into a car which is in full sunlight for some hours can reach temperatures of 80° C. If the Tg is too low the toner remelts at those elevated temperatures, and layers of the printed substrate material may stick to each other. Some storage stability problems after printing can be overcome by applying a varnish or lacquer but this is not always possible (e.g. office materials or books) and adds costs and complexity to the printing process.
At least for those reasons, many prior art toners have a higher Tg. However, for such toners, the amount of energy necessary to fuse the toner particle onto the substrate increases, making the use thereof energetically less interesting. Also, when using a higher Tg, a lot of substrates cannot be used anymore because of the limited substrate dimensional stability at higher fusing temperatures.
Taking into account the above, printer manufacturers are looking for a good balance between storage stability of the toner before printing, stable toner properties during the printing process, energy efficient printing, and storage stability of the printed substrates after printing